Paper products and method of making

ABSTRACT

Coated or uncoated paper that has small sized calcium carbonate on its surface. The calcium carbonate is placed on the paper web at the size press, by a spray or at a coating operation. The calcium carbonate can have a maximum mean average size of 200 nm. Another embodiment has a maximum mean average size of 100 nm. Another embodiment has a maximum mean average size of 50 nm. Different weight levels may be placed on the paper.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our copending patentapplications Ser. Nos. 10/744,926 and 10/744,861 both entitled “PaperProducts and methods of making” and filed December 22, 2003.

FIELD

The present invention is directed to printing paper, and to compositionsand methods for making it.

BACKGROUND

Hardwood and softwood wood pulp fibers are used in the manufacture ofprinting paper and newsprint. These fibers are produced in a chemicalpulping process, either sulfate or sulfite, or in a mechanical pulpingprocess. Mechanical processes would include thermomechanical andchemithermomechanical. To form the printing paper or newsprint, thesehardwood or softwood wood pulp fibers and wet end chemicals are mixedwith water in the headbox of the paper machine to form a suspension offibers and chemicals. The wet end chemicals may include fillers such ascalcium carbonate and clay. Both of these chemical would have a meanaverage diameter of one micron or more. Other wet end chemicals would beinternal sizes, opacifiers, brighteners, and dyes.

The suspension of fibers and chemicals flow from the headbox onto awire. The water is removed from the fibers and chemicals by both gravityand vacuum to form a wet web of pulp fibers into which the chemicals areincorporated. The chemicals are throughout the sheet. The sheet may bepressed and/or dried to remove more water.

Starch, optical brightener additives and surface size may be placed onthe surface of the sheet in a surface sizing step at the size press Thematerials that can be placed on the web at the size press must have aviscosity which allows the transfer of the material onto the web. Someof the materials may enter into the web if the pressure of the nip atthe press is great enough.

Thereafter the web of fiber, wet end chemicals and other materials isdried by heat and calendered and rolled into rolls. The resultingproduct is referred to as an uncoated paper sheet or web.

The uncoated sheet is coated in another application of one or morecoating layers placed on the sheet in an off-line coating operation. Theuncoated sheet passes through a second coating station and a seconddrying station. This paper sheet or web is referred to as a coated papersheet or web.

Uncoated or coated printing paper has a basis weight of from 16 to 180pounds per 3300 square feet of paper.

The paper may be printed with either aqueous inks or thermal or tonerinks. With either ink there is a concern about gloss variation and printvariation of the ink, and the density of the ink on the paper. It isdesired that the printing ink be evenly coated and have an even gloss.It is also desired that each of the different colored inks have an evendensity. It is also desired that the thermal toner adhere to the paperand not be easily removed.

In offset printing, ink from an ink roll is transferred to a printingplate. The printing plate has been treated so that ink is transferred tothe plate in the printing regions of the plate and not transferred tothe plate in the unprinted regions of the plate. The ink is transferredfrom the plate to a printing blanket which in turn transfers the ink tothe paper. During the process of transferring ink from the blanket tothe paper, fibers from the paper may attach and transfer to the blanket.This is called Tinting or picking. This is a problem because there is anunprinted section or void in the printed images following the transferof the fiber to the blanket because no ink is transferred where thefiber is attached to the blanket. The standard solution to the voidproblem is to stop the printing press at intervals during the press runand clean the blanket to remove the fibers. Many size press formulationshave been tried to prevent Tinting or picking. These formulations haveprovided limited improvement.

SUMMARY

In one embodiment the calcium carbonate has a mean particle size acrossthe particle of 200 nanometers (nm) or less. In another embodiment thecalcium carbonate has a mean particle size across the particle of 100 nmor less. In another embodiment the calcium carbonate has a mean particlesize across the particle of 15 to 50 nm.

In one embodiment the small sized calcium carbonate can be placed on theweb in amounts ranging from 0.1 to 300 pounds per ton of base paper. Inanother embodiment the small sized calcium carbonate can be placed onthe web in amounts ranging from 5 to 150 pounds per ton of base paper.In another embodiment the small sized calcium carbonate would be placedon the web in amounts ranging from 15 to 80 pounds per ton of basepaper. These weights would be divided between the two sides of paper. Ifthe small sized calcium carbonate is applied to only one side of thesheet then the weights would be from 2.5 to 75 pounds per ton of basepaper, or 7.5 to 40 pounds per ton of base paper, or up to 150 pound perton of base paper.

In one embodiment of the invention the small sized calcium carbonate isapplied at the size press or by a spray head before the dryer.

In another embodiment of the invention the small sized calcium carbonateis applied on an off-line coater.

In one embodiment the small sized calcium carbonate may be applied witha binder such as starch, modified starch or synthetic polymers orcopolymers.

The attributes of the paper will depend on the use to which the paper isput.

In one embodiment of the invention a printing paper sheet or web havinga small diameter calcium carbonate applied to its surface will have lessTinting or picking than a sheet that does not have such calciumcarbonate on its surface. This will allow more impressions to be madebefore the press is stopped and the printing plate cleaned and reducesthe cost and time of printing.

In another embodiment of the invention a printing paper sheet or webhaving a small diameter calcium carbonate applied to its surface will bestiffer than a sheet of the same weight that does not have such calciumcarbonate on its surface. This will allow a sheet to be used where paperstiffness is required for post printing and conversion operations.

In one embodiment of the invention a printing paper sheet or web havinga small diameter calcium carbonate applied to its surface will bettertoner adhesion than a sheet that does not have such calcium carbonate onits surface. The print will not be removed during use of the paper.

In another embodiment of the invention a printing paper sheet or webhaving a small diameter calcium carbonate applied to its surface willhave less print variance than a sheet that does not have such calciumcarbonate on its surface. The print will look better.

In another embodiment of the invention a printing paper sheet or webhaving a small diameter calcium carbonate applied to its surface willhave less gloss variance than a sheet that does not have such calciumcarbonate on its surface. The print will look better.

In another embodiment of the invention a printing paper sheet or webhaving a small diameter calcium carbonate applied to its surface willhave better color density than a sheet that does not have such calciumcarbonate on its surface. The print will look better.

In another embodiment of the invention an uncoated paper sheet or webhaving a small diameter calcium carbonate applied to its surface willhave many of the attributes of a coated paper web or sheet that does nothave such calcium carbonate on its surface. This will provide a lesscostly paper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a paper machine for uncoated paper.

FIG. 2 is a schematic diagram of a paper machine for coated paper.

FIG. 3 is a schematic cross sectional diagram of apparatus for obtaininga digital photograph for running the gloss variance and print variancetests.

FIG. 4 is a schematic diagram of apparatus for obtaining a digital imageused for the gloss variance values.

DETAILED DESCRIPTION

In one embodiment calcium carbonate having a mean particle size acrossthe particle of 200 nm (0.2 μm) or less is applied to the surface of apaper product. In another embodiment calcium carbonate having a meanparticle size across the particle of 100 nm (0.1 μm) or less is appliedto the surface of a paper product. In another embodiment the calciumcarbonate having a mean particle size across the particle of 15 nm(0.015 μm) to 50 nm (0.05 μm) is applied to the surface of a paperproduct. Throughout this application the term “small sized calciumcarbonate” is used. The term refers to the above embodiments of calciumcarbonate.

In one embodiment the small sized calcium carbonate is applied at thesize press of a paper machine. It has been found that small sizedcalcium carbonate has the appropriate viscosity to be placed on thepaper web at the size press. A portion of the small sized calciumcarbonate may enter the web because of the nip pressure of the sizepress but much will remain on the surface of the web.

In another embodiment the small sized calcium carbonate can be sprayedon the web prior to the dryer. The majority of this material will remainon the surface of the web.

In one embodiment the small sized calcium carbonate can be placed on theweb in amounts ranging from 0.1 to 300 pounds per ton of base paper. Inanother embodiment the small sized calcium carbonate can be placed onthe web in amounts ranging from 5 to 150 pounds per ton of base paper.In another embodiment the small sized calcium carbonate would be placedon the web in amounts ranging from 15 to 80 pounds per ton of basepaper. These weights would be divided between the two sides of paper. Ifthe small sized calcium carbonate is applied to only one side of thesheet then the weights would be from 2.5 to 75 pounds per ton of paper,or 7.5 to 40 pounds per ton of base paper, or up to 150 pound per ton ofbase paper.

In one embodiment the small sized calcium carbonate may be applied witha binder such as starch, modified starch or synthetic polymers orcopolymers.

FIG. 1 is a schematic drawing of a paper machine. Wood pulp fiberfurnish and wet end chemicals are mixed with water in a headbox 10 toform a slurry. The slurry exits the headbox through a slice 12 onto awire 14. The water in the slurry drains from the wire. A vacuum chest 16is also used to draw water from the slurry to form a wet paper web. Theweb is carried through press rolls 18 and a drier 20 that removeadditional water.

Additional size press chemicals or materials are placed on the wet paperweb at the size press 22. The size press may be a horizontal type withthe rolls horizontally aligned, or a vertical type with the rollsvertically aligned. The materials may be placed on the web from therolls or from a puddle between the rolls. The web may, in someinstances, be coated with material by the spraying apparatus 24. Thematerials described in the various embodiments in the presentapplication would also be applied at the size press 22 or the sprayingapparatus 24.

The paper web then passes through a drying section 26. The drying isusually done by steam heated drier cans through which the paper web isthreaded. The paper is then calendered by calender rolls 28 and rolledinto paper rolls at the winder 30. The resulting product is known asuncoated paper.

In the present invention the small sized calcium carbonate is added atthe size press 22 or at the spraying apparatus 24. The resultant paperhas better attributes than paper that is not coated with small sizedcalcium carbonate.

The small sized calcium carbonate, however, may also be placed on thepaper web or sheet at the coating station of a machine for making coatedpaper.

FIG. 2 is a schematic diagram of a paper machine for making coatedpaper. The reference numerals in FIG. 1 and 2 are the same for the sameelements. In FIG. 2 there is an additional off machine coatingoperation. The web goes from the dryer 26 to the coating operation andpasses through a coating station 32. Coating station 32 is shown asrolls but any type of coating equipment may be used. The web may thenpass through a dryer 34 and calender rolls 36. In some installationsthere are calender rolls before and after the coating station 32. Thepaper web is then wound into rolls 38.

The small sized calcium carbonate may be placed on the web at thecoating station 32 instead of size press 22 or spray head 24. In someinstances the small sized calcium carbonate may be placed on the web atthe size press 22 or spray head 24 and the coating station 32.

In one embodiment the application of small sized calcium carbonatereduces the voids that are found in the print surface. The followingtest is used to determine this.

Void Count Test

The void count test was run using a Diddie offset web press. A printingplate having a solid printing area of approximately 12.6 in² is used. Ithas a 0.9 ink density and a uses a 40% screen. A void is an area whichshould be printed but is not printed. It is caused by fibers coming offthe sheet surface and depositing on the printing plate or other printingsurface and blocking further printing where it is deposited. This iscalled picking or linting.

A paper roll containing the equivalent of at least 6000 8½″×11″ papersheets are run through the press using the same printing plate. Thenumber of voids in the 12.6 in² solid printed area of the sheet arecounted on the 1000^(th), 2000^(th), 4000^(th) 6000^(th) and 8000^(th)sheet.

In another embodiment the application of small sized calcium carbonateincreases the stiffness of paper. The following test is used todetermine this.

Cross Direction Gurley Stiffness

Cross direction Gurley stiffness of a paper sheet is determined usingTappi test method T-543 om-94. The bending resistance of paper isdetermined by measuring the force required to bend a sample undercontrolled conditions.

Toner Adhesion Test. This test method is used to determine the toneradhesion of papers imaged on a Xerographic copy machine, folded andcreased. A computerized image analysis is made of the crease in thesolid image area. An average pixel width is calculated and converted towidth in mm.

The equipment required for the test is a roller of specific weight anddimensions, cotton pads and a test sled.

The paper to be tested is conditioned at 50% relative humidity.

The copy machine is warmed up by running one ream of copy paper throughthe machine. 5% text is used.

A 5 block test pattern is used for the test. There is a pair of blocksat the top and a pair of blocks at the bottom of the test sheet. Each ofthe blocks is 3.8 mm×3.8 mm. There is a single block at the center ofthe test sheet. This block is 3.8 mm×7.5 mm. The copy machine is testedby printing 50 sheets with the test pattern and checking the density ofthe ink using a Gretag densitometer. The ink density should be between1.50 and 1.54. If the density is in the target range of 1.50 to 1.54then the samples can be printed. The samples should be in a 50% relativehumidity environment during printing. The ink density should bemonitored. If the density falls below 1.50 then printing of the samplesheets should be stopped. Additional copy paper should be printed untilthe density is above 1.50 and then printing of samples can be resumed.

The samples should be reconditioned at 50% relative humidity for 24hours after printing.

A sample is folded in the machine direction so that a crease line willfall in the middle of the sample. The sample is gently pressed in thefolded area without creasing the sample. The weighted roller is rolledgently over the folded area to create a crease. Use only the weight ofthe roller and a uniform and continuous movement in one direction toform the crease.

The paper is unfolded and a cotton pad is placed on the crease at oneend of the crease. A sled having a weight of 305 grams is placed on topof the pad and both are pulled the entire length of the crease in onecontinuous motion. The other side of the pad is placed on the crease atthe other end of the crease. The sled is placed on the pad and both arepulled along the crease I in the opposite direction.

A computerized image analysis is made of the crease in the solid imagearea. An average pixel width is calculated and converted to width in mm.

In the following examples, the small sized calcium carbonate wassupplied by NanoMaterials Technology Pte Ltd (NMT). It is a precipitatedcalcium carbonate made using the high gravity reactive precipitation(HGRP) technology platform.

EXAMPLE 1

In this example, calcium carbonate having a mean size of 40 nm wasapplied to the surface of the paper with the use of starch (PenfordGum-290). Formulations were made using 80-100 pounds ethylated starch(Penford Gum-290) per ton of paper and 0.5-180 pounds calcium carbonateper ton of paper. The amounts were pounds per ton based on the totalweight of the paper and calcium carbonate. Two types of calciumcarbonate were used. One was a standard coating grade of calciumcarbonate having a mean averages diameter of 1 micron. The other was acalcium carbonate having a mean average diameter of 40 nm. Theformulations were applied to a paper substrate using a laboratorytwo-roll press.

The substrate was a 70 pound per 3300 square feet paper web without sizepress starch and additives. The paper was prepared from a furnishincluding a blend of hardwood and softwood fibers and standardpapermaking additives such as wet end starch, sizing, calcium carbonate,optical brighteners and retention aids.

EXAMPLE 2

The papers from Example 1 were tested for Gurley stiffness using TAPPIMethod T543-om94. The results are summarized in Table 1. TABLE 1 eth- 1ylated micron 40 nm Cross- Change starch starch CaCO₃ CaCO₃ directionChange over 1 col- lbs/T lbs/T lbs/T lbs/T Gurley over micron umn paperpaper paper paper stiffness A % % A 80 119.1 B 80 15 116.9 80 15 139.2+16.8 +19.1 C 80 40 120.2 80 40 140.6 +18.1 +16.9 D 80 80 124.3 80 80137.2 +15.2 +10.4 E 80 100 130.2 80 100 176.1 +47.9 +35.3 F 80 150 132.080 150 167.7 +40.8 +27

It can be seen that paper sheets coated with small sized calciumcarbonate had an increase in cross-direction Gurley stiffness over thecontrol sheet at even at low levels of application, and increasedgreatly at higher levels of application. It can also be seen that smallsized calcium carbonate had a higher cross-direction stiffness thanconventional coating calcium carbonate at all levels of application.

EXAMPLE 3

In this example, calcium carbonate (NPCC-112) having a mean average sizeof 40 nm was applied to the surface of a paper web without the use of abinder such as starch, polyvinyl alcohol, or latex. Typically, a binderis required to attach calcium carbonate to the paper. The minimum ratiois 1 part binder to 1 part calcium carbonate.

The calcium carbonate was applied to the paper at 47 percent solids in atwo-roll laboratory pond size press. The resulting paper had much of thecalcium carbonate on its surface. The amount of calcium carbonateattached to the paper was at least 250 pounds per ton of paper.

EXAMPLE 4

Size press formulations were made using 40 pounds of ethylated starch(Penford Gum-290) per ton of paper per side. The formulations wereapplied to a paper substrate using a 2 roll size press and a gate roll.

The substrate was a 35 pound per 3300 square feet paper web without sizepress starch and additives. The paper was prepared from a furnishincluding a blend of hardwood and softwood fibers, and standardpapermaking additives such as wet end starch, sizing, standard calciumcarbonate, optical brighteners and retention aids.

EXAMPLE 5

Size press formulations were made using 80 pounds ethylated starch(Penford Gum-290) per 3300 square feet of paper per side of and twoweights of calcium carbonate having a mean average size of 40 nm. Thecalcium carbonate weights were 20 pounds per 3300 square feet of paperper side and 40 pounds per 3300 square feet of paper per side. Theformulations were applied to a paper substrate using a 2 roll sizepress.

The substrate was a 35 pound per 3300 square feet paper web without sizepress starch and additives. The paper was prepared from a furnishincluding a blend of hardwood and softwood fibers, and standardpapermaking additives such as wet end starch, sizing, standard calciumcarbonate, optical brighteners and retention aids.

EXAMPLE 6

Size press formulations were made using 80 pounds of ethylated starch(Penford Gum-290) per ton of paper per side. The formulations wereapplied to a paper substrate using a 2 roll pond size press.

The substrate was a 60 pound per 3300 square feet paper web without sizepress starch and additives. The paper was prepared from a furnishincluding a blend of hardwood and softwood fibers, and standardpapermaking additives such as wet end starch, sizing, standard calciumcarbonate, optical brighteners and retention aids.

EXAMPLE 7

Size press formulations were made using 80 pounds of ethylated starch(Penford Gum-290) per ton of paper per side and various weights ofcalcium carbonate having a mean average size of 40 nm. The calciumcarbonate weights were 7.5 pounds per ton of paper per side, 20 poundsper ton of paper per side, and 40 pounds per ton of paper per side. Theformulations were applied to a paper substrate using a laboratory 2 rollpond size press.

The substrate was a 60 pound per 3300 square feet paper web without sizepress starch and additives. The paper was prepared from a furnishincluding a blend of hardwood and softwood fibers, and standardpapermaking additives such as wet end starch, sizing, standard calciumcarbonate, optical brighteners and retention aids.

EXAMPLE 8

The papers from example 4, 5, 6, and 7 were tested for void count andcross-machine Gurley stiffness. The results are summarized in Table 2.TABLE 2 Cross small direction sized Gurley paper starch CaCO₃ Void VoidStiffness Lbs lbs/T lbs/T 6K 8K mg 35 80 46.5 35 80 40 4.0 35 80 80 7.060 80 47.0 127 60 80 15 15.0 138 60 80 40 7.0 10 165 60 80 80 12.0 13172

It can be seen that void count was reduced with the application of smallsized calcium carbonate. It can also be seen that cross machine Gurleystiffness increased.

EXAMPLE 9

In this example, calcium carbonate having a mean size of 40 nm wasapplied to the surface of the paper with the use of starch (PenfordGum-290). Formulations were made using 80 pounds ethylated starch(Penford Gum-290) per ton of paper and 0, 15, 40 or 80 pounds smallsized calcium carbonate per ton of paper. The amounts were pounds perton based on the total weight of the paper and calcium carbonate. Thestarch and calcium carbonate were evenly divided on both sides of thepaper. The formulations were applied to a paper substrate using alaboratory two-roll press.

The substrate was a 80 pound per 3300 square feet paper web without sizepress starch and additives. The paper was prepared from a furnishincluding a blend of hardwood and softwood fibers and standardpapermaking additives such as wet end starch, sizing, calcium carbonate,optical brighteners and retention aids.

EXAMPLE 10

The samples from Example 9 were printed and tested using the toneradhesion test. The results are given in Table 3. TABLE 3 Toner AdhesionCrack paper starch CaCO₃ Width Lbs lbs/T lbs/T mm 60 80 — 0.521 60 80 150.280 60 80 40 0.247 60 80 80 0.248

EXAMPLE 11

FIG. 3 shows the cross section of an apparatus used to obtain printswhich are used for the print variance values. Print variance is alsoknown as print mottle. The device 20 creates diffuse illumination usingan integrating sphere 22 onto which lights 24 shine. The diffuse lightshines onto a flat surface 26 holding the sample 28 centrally of thesphere 22. A digital camera 30 is aligned perpendicular to the samplesurface to capture a reflected light print image. A Kodak Megaplusdigital area camera with a 52 mm lens was used in this example. Thefield of view for this evaluation is 51.2×51.2 mm. The full digitalimage is captured in a single frame as an area image. The light level isset by setting the camera F-stop and adjusting the shutter speed untilthe average pixel intensity is approximately 127.

FIG. 4 shows an apparatus 40 used to obtain digital images which areused for the gloss variance values. Gloss variance is also known asgloss mottle. A sample 42 is placed onto a flat surface 44. Theapparatus creates directional illumination using a light 46 oriented atan angle to the surface of sample 42. The directional light shines ontoa flat surface of the sample 42. A digital camera 48 is on the side ofthe sample 42 opposite the light 46 and aligned with the light 46. Thecamera 48 captures the reflected light gloss image from the samplesurface. The camera 48 is oriented at the same angle as light 46 so thatthe illumination and detection angles are equal. As an example, both thelight 46 and camera 48 would be oriented at a 60° angle to the samplesurface. An EG&G Reticon digital line camera with a 105 mm lens was usedin this example. The field of view for this evaluation was 51.2×51.2 mm.The sample is moved linearly under convergence of the light beam andcamera field so that the camera 48 captures individual lines which arereconstructed into an area image. The light level is set by setting thecamera f-stop and adjusting the scanning speed until the average pixelintensity is approximately 120.

The images of the print and gloss samples are then used to determineprint mottle and gloss mottle, respectively. The pixel intensity foreach of the pixels on the photo is read and stored. If necessary, an 8×8order polynomial regression is applied to the print images and a 1×4polynomial regression is applied to the gloss images. The mean intensityfor the entire regressed image is calculated. The mean intensity issubtracted from the individual regressed data to provide intensitydifference image data. The intensity difference image data is multipliedby the pixel resolution in millimeters to scale the variance results.The autocovariance function of the intensity difference image data iscalculated. The circular footprint Hanning window extending to theNyquist frequency is also calculated. A windowed autocovariance functionis calculated by multiplying the autocovariance function by the Hanningwindow extending to the Nyquist frequency. The full power density matrixis extimated from the windowed autocovariance function using a FastFourier Transform.

The image power is calculated by summing up the elements of the FastFourier Transform array. The image power is given in Table 4. Thevariance was better when small sized calcium carbonate was used. TABLE 4Gloss Print Var. Var. paper starch CaCO₃ Image Image Lbs lbs/T lbs/TPower Power. 60 80 — 4.17 0.190 60 80 15 3.40 0.135 60 80 40 3.08 0.15060 80 80 2.72 0.160

EXAMPLE 11

Paper samples was printed with black, cyan, magenta and yellow using anHP 990C printer. The density of the colors was evaluated using a Gretagdensitometer. The results are shown in Table 5. Density increased withincreasing amounts of small sized calcium carbonate. TABLE 5 HP HP HP HP990C 990C 990C 990C Gretag Gretag Gretag Gretag paper starch CaCO₃Densi. Densi. Densi. Densi. Lbs lbs/T lbs/T Black Cyan Magenta Yellow 6080 — 1.04 1.01 0.89 0.75 60 80 15 1.42 1.02 0.97 0.76 60 80 40 1.35 1.061.01 0.78 60 80 80 1.36 1.09 1.05 0.81

Those skilled in the art will note that various changes may be made inthe embodiments described herein without departing from the spirit andscope of the present invention.

1. A paper sheet having on the surface thereof calcium carbonate, said calcium carbonate having a maximum mean average size of 200 nm, said calcium carbonate being present in the amount of from 0.1 to 300 pounds per ton of base paper.
 2. The paper sheet of claim 1 in which said calcium carbonate is present in the amount of 5 to 150 pounds per ton of base paper.
 3. The paper sheet of claim 1 in which said calcium carbonate is present in the amount of 5 to 80 pounds per ton of base paper.
 4. The paper sheet of claim 1 in which said calcium carbonate has a maximum mean average size of 100 nm.
 5. The paper sheet of claim 4 in which said calcium carbonate is present in the amount of 5 to 150 pounds per ton of base paper.
 6. The paper sheet of claim 4 in which said calcium carbonate is present in the amount of 5 to 80 pounds per ton of base paper.
 7. The paper sheet of claim 1 in which said calcium carbonate has a mean average size of from 15 to 50 nm.
 8. The paper sheet of claim 7 in which said calcium carbonate is present in the amount of 5 to 150 pounds per ton of base paper.
 9. The paper sheet of claim 7 in which said calcium carbonate is present in the amount of 5 to 80 pounds per ton of base paper. 